(a) Field of the Invention
The present invention relates to a method of manufacturing gas-permeable material for packaging a deoxidization agent, a freshness-preserving agent, and other agents.
(b) Description of the Prior Art
Gas permeable materials have been used for packaging various agents, such as a drying agent, an insecticide, a deoxidization agent, a freshness-preserving agent, and the like. When these agents are used for preserving foods, the materials must be not only gas permeable, but also water-resistant, damage-resistant and hygienically safe.
The composite material consisting of a sheet of paper laminated with a perforated polyethylene film has been used for packaging a deoxidization agent. The gas permeability of the material is controlled by the ratio of the area occupied by the pores of the film to the entire surface area of the film (hereinafter called "pore ratio"). This material is, however, not resistant to water. Hence, when the bags made of the composite material and containing a deoxidization agent are used to preserve food of a high moisture content, they are damped, whereby the agent oozes out through the pores. In the worst case, the bags may be broken, and the agent may come into contact with the food. The composite material is not preferred from a hygienical point of view.
The gas permeability of the polyethylene-paper material cannot always be well controlled. During the thermal bonding of the polyethylene film and the sheet of paper, the pores are deformed, whereby the material fails to have the desired gas permeability. If the bags made of such material and containing a deoxidization agent, it is difficult to control the oxygen absorption rate.
Japanese Patent Disclosures No. Sho 53-51096 and No. Sho 53-50065 disclose a packaging material consisting of two perforated plastic films laminated together and a packaging material consisting of two perforated plastic films and a sheet of paper interposed between these films. These materials are more water-resistant than a sheet of paper. Their gas permeability is, however, impaired since an adhesive such as polyethylene is used to adhere the plastic films together, or to adhere the paper sheet to the plastic films. Hence, with these materials it is practically impossible to control the gas permeability by changing the pore ratio.
Another similar composite material for packaging a deoxidization agent is known. This is a laminate made by adhering a plastic film to a layer of adhesive such as polyethylene, perforating the resultant laminate and heat-sealing laminate to a perforated heat-sealable film with a sheet of paper interposed between the laminate and heat-sealable film. This composite material has a drawback. It cannot have a desired gas permeability since the pores of the laminate and heat-sealable film are deformed during the heat sealing, inevitably changing the gas permeability, or the chips made in perforating the laminate stick in the pores, adversely reducing the gas permeability.
Japanese Patent Disclosure No. Sho 56-124440 discloses a packaging material made by preparing two perforated laminates each consisting of a plastic film and a sheet of paper, and by adhering the paper sheets of the laminates are bonded with an adhesive. It is also extremely difficult with this packaging material to control the gas permeability, since a layer of the adhesive such as ethylene-vinyl acetate copolymer is interposed between the paper sheets.
Japanese Patent Disclosure No. Sho 56-124441 also discloses a packaging material comprised of two laminates. One of the laminates consists of a plastic sheet and a sheet of paper, and is perforated. The other laminate consists of a plastic film having a gas permeability of 3000 ml/m.sup.2 24 Hr. atm. or more, and a sheet of paper. The paper sides of these two laminates are faced and sealed each other with an adhesive layer interposed therebetween to integrally form a packaging material. With this packaging material it is also hard to control the gas permeability since a layer of the adhesive such as ethylene-vinyl acetate copolymer.
In summary, with the conventional packaging materials it is extremely difficult to control the gas permeability accurately.